The present invention relates to color image forming equipment and, more particularly, to a digital color copier of the type using a developer made up of a toner and a carrier, i.e., a two-component developer.
A prerequisite with a digital color copier of the type described is that the toner concentration of the two-component developer be adequately regulated to enhance the reproducibility of tones, especially halftone, of images. To meet this requirement, various toner concentration control methods have heretofore been proposed. The conventional methods may generally be classified into two types, as follows:
Type A: sensing toner concentration or a substitute characteristic and controlling it to a predetermined one; and
Type B: sensing the developing ability of a developer or a substitute characteristic and controlling toner concentration such that the developing ability remains constant.
The type A method consists in, for example, detecting changes in the volume density of a developer (Japanese Patent Laid-Open Publication No. 5487/1972), detecting changes in the volume density of a developer in terms of changes in magnetic permeability or reactance (Japanese Patent Laid-Open Publication No. 5138/1972), detecting changes in the volume of a developer (Japanese Patent Laid-Open Publication No. 19459/1975), detecting changes in the volume of a developer in terms of changes in torque (Japanese Patent Laid-Open Publication No. 6598/1972), detecting changes in the tone of a developer (Japanese Patent Laid-Open Publication No. 69527/1973), detecting changes in the electric resistance of a developer (Japanese Patent Laid-Open Publication No. 38157/1973), or detecting a voltage induced by the counter charge (on a carrier) of a developed toner (Japanese Patent Laid-Open Publication Nos. 57638/1973 and 42739/1973). Type-B methods include formation and developement of a charge pattern which is invariant with any photoconductive body, so as to optically sense the density of a resulting toner image.
Such a prior art method, whether it be of type A or type B, cannot satisfactorily reproduce halftone images. Specifically, toner concentration generally changes with the ambient conditions and due to aging. Hence, the type A method which maintains toner concentration constant causes the developing characteristic of the developer to change due to changes in ambient conditions and aging. This type of method, therefore, is not directly applicable to a color copier which attaches importance to the reproducibility of halftone. In the light of this, there have also been proposed a control method which controls the quantity of exposing light by sensing ambient conditions as well as other factors (Japanese Patent Laid-Open Publication No. 177153/1988), and a control method which develops a plurality of potential patterns, optically senses the densities of the resulting toner images, and selects adequate one of exposing potential data which were measured in various environments (Japanese Patent Laid-Open Publication No. 296061/1988). These methods, however, cannot cope with changes in the charging characteristic of a developer due to aging. Although they will be capable of coping with such changes if provided with data covering both the aging and the ambient conditions, preparing such an amount of data is not practical. Moreover, optimizing the developing characteristic by any of the above-mentioned methods is almost impracticable since toner concentration is susceptible to operation modes as well as to aging and ambient conditions.
The type A method is not satisfactory not only from the standpoint of the above-discussed optimization of developing characteristic but also from the standpoint of adequate toner concentration. Specifically, the limit of toner concentration at which the contamination of background and the scattering of toner sharply increase is also susceptible to changes in ambient conditions and aging. It follows that controlling the toner concentration to a predetermined one as with the type A method is apt to bring about the contamination of background and the scattering of toner due to changes in ambient conditions and aging. As a result, even when the developer is still usable, it is often determined that it should be replaced with fresh one. Concerning the type B method which so controls the toner concentration as to maintain the developing ability constant, all the changes in the developer ascribable to the environment and aging are fed back to the toner concentration, broadening the range over which the toner concentration is varied. Consequently, the developing ability of the developer is increased in a high humidity environment or in an aged condition. In this condition, should the toner concentration be reduced to control the developing ability to a usual one, the resulting toner concentration would be excessively low to in turn reduce the maximum amount of development, i.e., saturation image density. For this reason, the halftone reproducibility achievable with the type B method is as poor as the type A method.
We have already proposed control methods capable of eliminating the above problems in copending U.S. patent application Ser. No. 07/545,508 filed Jun.29, 1990 and Japanese Patent Application No. 238107/1989. With such methods, it is possible to achieve a stable image density, especially halftone reproducibility, despite the changes in ambient conditions and aging.
Regarding the control over the toner supply, it sometimes occurs that the amount of toner deposition on the photoconductive element abruptly increases due to, among others, the change in the characteristic of the developer ascribable to the ambient conditions and other factors. Especially, in the case of a color toner, since a diffused reflection from the toner deposited on the photoconductive element and, therefore, the quantity of light incident to an optical sensor increases with the increase in the amount of toner deposition. Therefore, the amount of toner deposition is apt to be erroneously determined as being low. Then, the toner supply would be continued to increase the toner density to an excessive degree. To eliminate such an occurrence, it has been customary to control the toner supply on the basis of a relation between the background voltage Vsg+matching the quantity of a reflection from the photoconductive element detected when the developing sleeve was halted and the photoconductive element was in operation, and the background voltage Vsg detected when the developing sleeve was in operation. Specifically, when Vsg+&lt;Vsg, whether (I) Vsp (target value).times.4/Vsg&gt;Vsp or (II) Vsp.times.4/Vsg+&lt;Vsp is determined. The toner is supplied only if the condition (II) is satisfied.
However, the problem with the above-stated scheme is that when the voltage Vsg detected when the developing sleeve was in operation becomes higher than the voltage Vsg+detected when it was in a halt, the condition (II) holds to cause the toner to be continuously supplied. This is also apt to result in the uncontrollable increase in toner concentration. Specifically, the reversal of the relation between the detected voltages of interest stems from the fact that the amount of toner remaining on the background increases when, for example, the photoconductive element is not completely cleaned or when the toner density is increased to an unusual degree. Besides, the control over the toner density often fails when an error occurs in the image forming system including the photoconductive element.